As West Nile Virus (WNV) continues to move into new areas and infection of large numbers of non-immune individuals remains probable, the possible adverse effect of WNV infection on outcomes of pregnancy is of concern. Little is known on the prevalence of WNV infection in pregnant women and the impact of WNV infection on pregnancy outcomes. Because of immunosuppresion, pregnant women may be prone to more severe sequelae from WNV infection. The developing fetus may also be at risk for infection. The investigators hypothesize that WNV in pregnancy, through direct transplacental intrauterine infection or by indirectly causing severe maternal conditions, increases the risk of adverse pregnancy outcomes such as birth defects, abortion, preterm birth, intrauterine growth restriction, low birth weight, and neonatal neurological impairment. The first aim of the proposed research will be to enroll women infected with WNV during pregnancy in a prospective study of congenital infection. The investigators will enroll into a prospective study all WNV-infected women identified by CDC. Non-exposed controls will be recruited from women delivering in the same hospitals as WNV-infected women and matched for key potential confounders, which are zip code or hospital of delivery and maternal age. Cases and controls will be followed up, and information regarding maternal, newborn, and infant outcomes will be obtained. Incidence of adverse pregnancy outcomes will be compared between the cohort of women infected with WNV and the non-infected control group. A web-based data management system will be developed to assist in data collection and reporting. The second aim will be to evaluate the effect of WNV infection on pregnancy outcomes. The investigators will collect information on outcomes as recommended by the interim guidelines published in the Morbidity and Mortality Weekly Report (MMWR), including infant outcomes and maternal pregnancy complications. The third aim will be to conduct an in-depth examination of placenta, decidua, and fetal membranes associated with infants born to WNV-infected mothers or with evidence of congenital infection. The investigators will perform morphological, endocrine, and molecular assessments of the villous placenta, trophoblast, conceptus membranes, and maternal deciduas, as these structures are of vital importance to both optimal fetal development/growth, and the prevention of spontaneous abortion/premature labor. The performance site for the proposed research is Tulane University. The principal investigator is Dr. Dawn Wesson, Associate Professor of Tropical Medicine, who is a medical entomologist and an expert in the ecology of West Nile Virus. Other investigators for the proposed research include an infectious disease epidemiologist (Dr. Patricia Kissinger), a perinatal epidemiologist (Dr. Xu Xiong), a perinatal and pediatric epidemiologist (Dr. Pierre Buekens), an expert in epidemiology and diagnosis of birth defects (Dr. Paul Romitti from University of Iowa), an expert in perinatal research and effects of infectious diseases on pregnancy (Dr. Michael Henson), and an expert in maternal fetal medicine and clinical genetics (Dr. Gabriella Pridjian). The first year of the proposed research will focus on establishing the web-based data collection system, working with CDC to bring all retrospective cases into the database, identifying controls for retrospective cases, recruiting and collecting information on prospective cases and identifying controls, collecting cord blood and culturing placental samples. Years 2-4 will be spent collecting data on prospective cases and controls, collecting cord blood and placental samples, conducting follow-up interviews and data collection on established cases and controls, assaying cord blood and placental samples, culturing placental samples, and convening the adjudication committee to review the data on completed cases. The 4th year will also focus on data analysis.